How to find the port size for a sub box

Written by jonathan shaffer
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How to find the port size for a sub box
A port tube allows air to escape the speaker enclosure. (3 way Speaker image by Edward Stephens from

The port tube in a subwoofer enclosure allows air to escape the enclosure while the speaker drivers create air pressure. The low end that you feel coming from your subwoofer is a direct result of this air flow. If a port tube is too short for its enclosure, too much air will be released at once and result in a loss of pressure and overall tone. Finding the correct length for your port tube can make the difference in a good sounding subwoofer and a great sounding subwoofer.

Skill level:

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Things you need

  • Port tube
  • Chop saw
  • Measuring tape
  • Pencil
  • Port tube length formula

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  1. 1

    Visit one of the three online port tube length formula calculators in the References section. These calculators will help you find the appropriate length for your port tube based on the size of your enclosure and the diameter of your port hole. Fill in the required information in one of the sites, and calculate your port length. This is the number of inches that you need to cut your port tube down to.

  2. 2

    Measure your port tube, and mark the point at which you need to cut it.

  3. 3

    Cut the port tube down to size using your chop saw. The tube can now be installed and will be the optimal length to work with your speaker enclosure.

Tips and warnings

  • Always use caution when cutting with a chop saw, because it can result in bodily harm.

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